Catalyst formed by spraying a titanium hydroxide material

ABSTRACT

A catalyst includes a catalytically active composition produced by thermal spraying of a spraying material onto a support body. The spraying material includes a titanium hydroxide reactive precursor of at least one component of the catalytically active composition, and the titanium hydroxide reactive precursor converts to form the at least one component.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This is a division of U.S. application Ser. No. 09/217,854, filedDec. 21, 1998, which was a continuation of copending InternationalApplication No. PCT/DE97/01161, filed on Jun. 9, 1997, which designatedthe United States.

BACKGROUND OF THE INVENTION

[0002] Field of the Invention

[0003] The invention relates to a catalyst formed by spaying a titaniumhydroxide material.

[0004] U.S. Pat. No. 3,271,326 discloses an essentiallynickel-containing catalyst in which a catalytically active surface isapplied by flame spraying. In that case, in a first process step, asteel support structure which had previously been mechanically roughenedhas aluminum flame sprayed on to it for the purpose of furtherroughening. Subsequently, in a second process step, the catalyticallyactive components are applied by flame spraying to the support structurewhich was pretreated in that way. A characteristic of that complicatedproduction process is that the catalytically active components which aresprayed on are heated up to their melting point.

[0005] Furthermore, German Published, Non-Prosecuted Patent ApplicationDE 38 13 312 A1 discloses applying a titanium dioxide modified withcatalytically active components as additives to a metallic structure bythermal spraying. In that procedure, the catalytically active componentsmelt briefly and upon cooling fuse together at contact points. Thatcreates a catalytically active surface held together by adhesion forces.

[0006] However, it is known that the catalytic activity of the titaniumdioxide is very strongly dependent on the proportion of the titaniumdioxide which is present in the anatase modification. The anatasemodification of titanium dioxide in that case has the property of beingconverted irreversibly by high temperatures into the less active rutilemodification. Nuclei of titanium oxide in the rutile modification in thecrystal microstructure grow further upon heating at the expense of theanatase modification.

[0007] On that subject, German Published, Non-Prosecuted PatentApplication DE 39 16 398 A1 discloses mixing a very low-meltingcomponent into the spraying material containing a thermally sensitivecomponent such as titanium dioxide in the anatase modification. Themelting point of the very low-melting component in that case is below aninactivation temperature above which, upon influence by a chemicalchange, for example, the amount of the thermally sensitive component isirreversibly reduced. During thermal spraying, the spraying material isheated only to the melting point of the very low-melting component, sothat inactivation of the thermally sensitive component is avoided. Theadhesion of the components of the spraying material to one anotheroccurs due to the very low-melting component flowing around otherparticles and fusing together during the thermal spraying. That enablesa surface having a high catalytic activity to be achieved by thermalspraying, even in the presence of a thermally sensitive component.

[0008] However, a very low-melting component flowing around acatalytically active component reduces the specific surface area or BETsurface area of the composition being applied. In order to achieve ahigh catalytic activity, the layer thickness of the catalytically activecomposition being applied accordingly has to be increased. The longerspraying time resulting therefrom can then easily lead to thermaldistortion of the support body or the support structure. The process istherefore only suitable for support structure thicknesses of more thanabout 3 mm.

SUMMARY OF THE INVENTION

[0009] It is accordingly an object of the invention to provide acatalyst formed by spraying a titanium hydroxide material, whichovercomes the hereinafore-mentioned disadvantages of theheretofore-known processes and products of this general type, in whichthe catalyst includes a catalytically active composition on a supportbody, in which the catalyst is produced by thermal spraying withoutthermally sensitive components becoming inactivated and in which thecatalytically active composition has a significantly increased BETsurface area as compared with the prior art.

[0010] With the foregoing and other objects in view there is provided,in accordance with the invention, a process for producing a catalystwith a catalytically active composition on a support body by thermalspraying, which comprises thermally spraying a spraying materialincluding a titanium hydroxide reactive precursor of at least onecomponent of the catalytically active composition onto the support body,and converting the titanium hydroxide reactive precursor to form thecomponent.

[0011] In accordance with another mode of the invention, the thermalspraying step is carried out with a titanium metahydroxide as thetitanium hydroxide.

[0012] The invention is based on the concept, that is exactly contraryto the opinion of those skilled in the art, of selecting a compositionfor the spraying material which is not the same as that for thecatalytically active composition to be applied to the support body.Rather, for at least one component of the catalytically activecomposition, a reactive precursor which is selected in the sprayingmaterial is converted to form the component. In this way, for example inthe case of a thermally sensitive component such as titanium dioxide,the inactivation favored by a temperature rise during the sprayingprocess can be avoided. This is because the component is only producedfrom the reactive precursor during or after the spraying process and asa result cannot be inactivated by a temperature rise occurring beforeconversion.

[0013] It has been found that a catalytically active compositionproduced by the process of the invention has a high BET surface area offrom 50 to 70 m²/g. According to comprehensive studies, it can beassumed that during the conversion of the reactive precursor linking ofchemical compounds occurs which lead to the formation of interconnectedmicrocrystallites. Such an assembly of connected microcrystallites has ahigh specific surface area. The high BET surface area enables the layerthickness of the catalytically active composition to be considerablyreduced as compared with the prior art while maintaining an equally highcatalytic activity. As a result of the shorter spraying time associatedtherewith, a support body having a thickness of less than 100 μm issuitable for the application of the catalytically active compositionwithout thermal distortion occurring during application. Such a materialsaving allows the manufacturing costs to be reduced correspondingly.Suitable support bodies are metallic or ceramic bodies of any shape,e.g. in the form of a plate, a band, a rod or a tube. It is alsoconceivable to use a material other than metal or ceramic for thesupport body, as long as it cannot be damaged by the elevatedtemperature during the spraying process.

[0014] In accordance with another mode of the invention, the conversionor chemical reaction of the reactive precursor is carried out by thermalactivation during spraying. The spraying material in this case is heatedduring the thermal spraying to above a corresponding activationtemperature above which a chemical reaction of the precursor commences.

[0015] In accordance with a further mode of the invention, the thermalactivation of the reactive precursor can also be carried out afterspraying is completed by heat treatment of the catalytically activecomposition applied or the support body at above the activationtemperature. Such a heat treatment can also include a calcinationprocess.

[0016] In accordance with an added mode of the invention, a suitablereactive precursor convertible by thermal activation is, in particular,a readily thermally decomposable metal salt or a compound of a metalbearing a hydroxy group (hydroxy compound). A metal salt or an ioniccompound of the corresponding metal can be ionized by appropriate heatinput into a cation and an anion. Such an ionization takes place, forexample, in any candle flame. If the temperature during thermalspraying, which for the purposes of the present invention includes bothplasma and flame spraying, is selected so as to be appropriately high, afree metal ion can react with a gas molecule of the surroundingatmosphere and, for example, with oxygen to form a metal oxide as adesired component. For the purposes of the present invention, a hydroxycompound of a metal is a not yet completely dewatered, i.e. stillcontaining OH groups, oxide compound of the metal. Such compounds can beeasily converted thermally into the corresponding oxide withelimination, if necessary even multiple elimination, of water. In thisway, a hydroxy compound of a metal can be converted into a metal oxidethrough the use of an appropriately selected temperature during thespraying process.

[0017] In accordance with an additional mode of the invention, the metalsalt being used is an oxalate, a nitrate or a carbonate. Such a metalsalt can be ionized particularly easily, i.e. at a temperature of lessthan 500° C.

[0018] In accordance with yet another mode of the invention, thereactive precursor is an aluminum hydroxide, preferably a gibbsite(monoclinic γ-Al(OH)₃) or a boehmite (rhombic crystalline metahydroxideγ-AlO(OH)), or a titanium hydroxide, preferably a titanium metahydroxideTiO(OH)₂, also known as metatitanic acid. Both the aluminum hydroxideand the titanium metahydroxide can be easily converted into thecorresponding oxide form through the use of relatively hightemperatures. The titanium dioxide TiO₂ produced by the thermalactivation of the titanium metahydroxide TiO(OH)₂ is a main constituentof many catalysts. In particular, a catalyst containing titanium dioxideis particularly suitable for removing nitrogen oxides through the use ofthe known DeNO_(x) process.

[0019] The catalytically active composition applied by thermal sprayinghas a particularly high BET surface area if the spraying material beingused includes a plurality of reactive precursors. For example, titaniummetahydroxide TiO(OH)₂ can be used as a reactive precursor for thecatalytically active component titanium dioxide TiO₂, a boehmite and/ora gibbsite can be used for aluminum oxide Al₂O₃, and an oxalate can beused as respective reactive precursor for further catalytically activecomponents. Appropriate heating of the spraying material during thermalspraying leads to increasing elimination of water from the hydroxycompounds. The elimination of an OH group from metatitanic acid and aproton from aluminum hydroxide can easily result in formation of aternary oxide or mixed oxide of aluminum and titanium. The otherreactive precursors are ionized and, for example, converted into theiroxides in an oxygen-containing atmosphere. In this way, a highlycatalytically active composition can be produced on a support body bythermal spraying.

[0020] As already mentioned, titanium dioxide can exist both in a rutileand in an anatase modification. The anatase modification has asignificantly increased catalytic activity as compared with the rutilemodification.

[0021] In accordance with yet a further mode of the invention,preferential crystallization of the favorable modification occurs when acoprecipitate is mixed into the spraying material. For the purposes ofthe present invention, coprecipition is a precipitation of a chemicalelement or a chemical compound in the presence of materials which aresoluble. A coprecipitate is accordingly a precipitate of a chemicalelement or a chemical compound mixed with another material. For example,a titanium dioxide mixed with tungsten can be obtained from a solutioncontaining titanyl sulfate and paratungstate. The tungsten atoms in thiscase are incorporated into intersitial sites of the titanium dioxidelattice. Drying and calcination of this coprecipitate gives a titaniumdioxide crystallized within the anatase modification, with theintercalated tungsten atoms acting as a block for the phasetransformation into the rutile modification. A further coprecipitate is,for example, a crystalline mixture of metatitanic acid and tungstic acid(TiO (OH)₂/WO (OH)₂) . Such a coprecipitate can prevent a phasetransformation into an unfavorable crystalline modification fromoccurring at all during conversion of a reactive precursor into acatalytically active component.

[0022] In accordance with yet an added mode of the invention, thethermal spraying is carried out in an oxygen-containing atmosphere forthe production of a catalyst containing a metal oxide. In this case, afree metal ion (formed by the ionization of a salt) combines with oxygento give a metal oxide.

[0023] In accordance with yet an additional mode of the invention, ametal or a metal alloy is sprayed in parallel with the sprayingmaterial, with the metal or the metal alloy and the spraying materialbeing intimately mixed during spraying before they impinge on thesupport body. In this case, the metal or the metal alloy acts, asdescribed in the introduction in the acknowledgement of the prior art,as a composite material. During the spraying process, the softened metalor the softened metal alloy flows around the other components andcontributes to their adhesion to one another and to the support body.Particularly suitable metals or metal alloys are aluminum and aluminumalloys.

[0024] In accordance with again another mode of the invention, aspraying material which is particularly readily handleable for thermalspraying is in the form of a powder mixture made from separate powderseach having a mean particle size of less than 50 μm, preferably lessthan 10 μm. The separate powders are intensively mixed prior to thespraying process. The metal or the metal alloy can equally well besprayed separately. The small particle size that is used enables a goodconversion of the reactive precursors and an increase in the BET surfacearea of the catalytically active composition which is applied to beachieved.

[0025] With the objects of the invention in view there is also provideda catalyst, comprising a catalytically active composition produced bythermal spraying of a spraying material onto a support body; thespraying material including a titanium hydroxide reactive precursor ofat least one component of the catalytically active composition; and thetitanium hydroxide reactive precursor having been converted before orafter spraying to form the at least one component.

[0026] In accordance with another feature of the invention, thecatalytically active composition includes multinary compounds. For thepurposes of the present invention, a multinary compound is a complex ormixed compound of a plurality of components of the catalytically activecomposition. Such a compound contributes substantially to improvedadhesion of the individual components to one another. If, for example, afirst reactive precursor being used is a vanadium oxalate and a secondreactive precursor is a metatitanic acid, then a multinary compound inthe form of a mixed metal oxide containing oxygen and both vanadium andtitanium can be formed during thermal spraying in an oxygen-containingatmosphere.

[0027] In accordance with a further feature of the invention, thecatalytically active composition has a BET surface area of from 40 to100 m²/g, preferably from 50 to 70 m²/g. Such a high BET surface areaenables a high catalytic activity of the catalyst to be achieved evenwith a low layer thickness of the catalytically active composition.Since a reduced layer thickness of the catalytically active compositionis linked to a shorter spraying time, a support body having a lowthickness can also be used without distortion of the support bodyoccurring during thermal spraying.

[0028] In accordance with an added feature of the invention, the supportbody has a thickness of less than 1 mm, preferably less than 100 μm.

[0029] The support body itself can be formed of a metal or a ceramic. Inthis case the support body can have any desired structure, e.g. in theform of a plate, a band, a rod or a tube. The support body can also havea honeycomb structure.

[0030] In accordance with a concomitant feature of the invention, thematerial for the support body is a chromium-aluminum steel. Such asupport body enables a high operating life of the catalyst to beachieved.

[0031] In order to obtain improved adhesion of the catalytically activecomposition, the support body can be mechanically or chemicallyroughened prior to application of the catalytically active composition.

[0032] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0033] Although the invention is illustrated and described herein asembodied in a catalyst formed by spraying a titanium hydroxide material,it is nevertheless not intended to be limited to the details shown,since various modifications and structural changes may be made thereinwithout departing from the spirit of the invention and within the scopeand range of equivalents of the claims.

[0034] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The FIGURE of the drawing is a diagrammatic, cross-sectional viewof a support body of a catalyst of the invention and components of acatalytically active composition applied thereto by thermal spraying.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Table 1 shows possible components of a spraying material forproducing a DeNO_(x) catalyst, with a respective percentage by mass anda respective mean particle size being indicated.

[0037] Table 2 shows four alternative compositions of a sprayingmaterial for producing a DeNO_(x) catalyst, with the components shownbeing taken from Table 1.

[0038] In the case of the illustrative embodiments, which concerncatalysts having oxidic catalytically active components, the thermalspraying takes place in an oxygen-containing atmosphere. The temperatureduring the thermal spraying is selected in such a way that the thermalactivation of the reactive precursor is complete before it impinges onthe support body. As a final step, the composition which is applied iscalcined. This achieves complete dewatering and a final oxidic structureof the catalytically active composition. The spraying material that isused is always a powder mixture of separate powders of the respectivecomponents. The mean particle sizes of the individual components of thespraying material are shown in Table 1. TABLE 1 Component Percentage byMass Mean Particle Size Aluminum  8.0% <30 μm Aluminum alloy  8.0% <30μm (AlMg₃) Boehmite 26.0% <10 μm Gibbsite 16.5% <10 μm Vanadium oxalate 4.0% <10 μm Tungsten oxalate 3.75% <10 μm Coprecipitate 37.5% <1 μm(TiO₂/WO₃) Coprecipitate 37.5% <1 μm (TiO(OH)₂/H₂WO₄) TiO(OH)₂ 33.75% <10 μm

[0039] Table 1 shows, for a spraying material in the form of a powdermixture, suitable components for producing a DeNO_(x) catalyst for thedegradation of nitrogen oxides in a waste gas of a combustion plantusing a reducing agent such as ammonia.

[0040] Table 2 shows the respective percentages by mass (column 2) andthe respective mean particle sizes (column 3). Separate parallelspraying of the aluminum or aluminum alloy, in this case includingaluminum and manganese, is carried out. Mixing with the other componentsoccurs before they impinge on the support body. The remaining componentsare mixed to form a spraying material before thermal spraying. It ispossible to use either a metatitanic acid or a mixture of tungstic acidwith a metatitanic acid in the form of a coprecipitate, as a reactiveprecursor for the titanium dioxide which is catalytically active in aDeNO_(x) catalyst. It is also possible to use a dried and calcined jointprecipitate of titanium and tungsten from a solution containing titanylsulfate and paratungstate, as such a coprecipitate. In thiscoprecipitate, the titanium dioxide is predominantly in thecatalytically active anatase modification, with the tungsten beingincorporated in interstitial lattice sites. The mixing in orincorporation of tungsten prevents the conversion of titanium dioxidefrom the anatase modification into the undesired rutile modification.TABLE 2 Composition Example 1 Example 2 Example 3 Example 4 Aluminum X XX Aluminum alloy X Boehmite X X X X Gibbsite X X X X Vanadium oxalate XX X X Tungsten oxalate X Coprecipitate X (TiO₂/WO₃) Coprecipitate X X(TiO(OH)₂/H₂WO₄) Metatitanic acid X (TiO(OH)₂)

[0041] Table 2 shows four alternative possible compositions of aspraying material for producing a DeNO_(x) catalyst. The percentage bymass for each component is shown in Table 1. In each case, aluminum(Example 1 to Example 3) or an aluminum alloy including aluminum andmanganese, customarily designated as AlMg₃ (Example 4), is sprayed inparallel with a mixture of the other components. The thermal activationof the reactive precursors (in this case: boehmite, gibbsite, vanadiumoxalate, tungsten oxalate, metatitanic acid and the coprecipitateincluding a mixture of tungstic acid and metatitanic acid) occurs duringthermal spraying. The composition which is applied is subjected to acalcination process to achieve final dewatering and the catalyticallyactive oxidic structure of the composition that is applied. All of thecatalytically active compositions produced according to Examples 1 to 4have a BET surface area of from 60 to 70 m²/g.

[0042] An X-ray structure analysis demonstrates the advantage of theprocess of the invention. If a reactive precursor in the form ofmetatitanic acid or the coprecipitate containing metatitanic acid isused for the catalytically active component titanium dioxide, thetitanium dioxide present in the catalytically active composition ispredominantly in the anatase modification. The thermal inactivation ofthe anatase modification by conversion into the rutile modification canbe effectively avoided, since the reactive precursor is convertedprimarily into titanium dioxide in the anatase modification. Phasetransformation into the rutile modification does not take place. Thesituation is different if a titanium dioxide containing tungstenincorporated in interstitial lattice sites is used for the sprayingmaterial. Such a coprecipitate is not a reactive precursor for thepurposes of the invention. No conversion takes place.

[0043] Referring now in detail to the single FIGURE of the drawing,there is seen a cross-section through a catalyst produced according toExample 1, for the degradation of nitrogen oxides by the DeNO_(x)process. A support body 1 is a chromium-aluminum steel in the form of aplate having a thickness of 40 μm. A catalytically active composition 10is applied on both sides by thermal spraying. The surface of the supportbody 1 in FIG. 1 has not been given any particular configuration, but itcan, for example, be roughened by mechanical or chemical treatment. As aresult of deformation on impact, aluminum oxide 2 adheres to the supportbody 1 due to adhesion forces. Aluminum 3 which is sprayed in parallelacts as a composite material, it links the individual catalyticallyactive components both to one another and it also links thecatalytically active composition 10 to the support body 1. Thecatalytically active components titanium dioxide (TiO₂) 4, vanadiumpentoxide (V₂O₅) 5 and tungsten trioxide (WO₃) 6 are disposed aroundeach microcrystallite of the aluminum oxide 2. In addition to theadhesion forces, the respective catalytically active components 4, 5, 6and the aluminum oxide 2 are held together by chemical bonds due to theformation of ternary oxides. Such mixed oxides lead to a high abrasionresistance of the catalytically active composition 10. A long operatinglife of such a catalyst is associated therewith.

We claim:
 1. A catalyst, comprising: a catalytically active compositionproduced by thermal spraying of a spraying material onto a support body;said spraying material including a titanium hydroxide reactive precursorof at least one component of said catalytically active composition; andsaid titanium hydroxide reactive precursor converted to form said atleast one component.
 2. The catalyst according to claim 1 , wherein saidtitanium hydroxide reactive precursor is converted before spraying toform said at least one component.
 3. The catalyst according to claim 1 ,wherein said titanium hydroxide reactive precursor is converted afterspraying to form said at least one component.
 4. The catalyst accordingto claim 1 , wherein said catalytically active composition includesmultinary compounds.
 5. The catalyst according to claim 1 , wherein saidcatalytically active composition includes ternary metal oxides.
 6. Thecatalyst according to claim 1 , wherein said catalytically activecomposition has a BET surface area of from 40 to 100 m²/g.
 7. Thecatalyst according to claim 1 , wherein said catalytically activecomposition has a BET surface area of from 50 to 70 m²/g.
 8. Thecatalyst according to claim 1 , wherein said support body has athickness of less than 1 mm.
 9. The catalyst according to claim 1 ,wherein said support body has a thickness of less than 100 μm.
 10. Thecatalyst according to claim 1 , wherein said support body is formed of achromium-aluminum steel.